MICROSTRUCTURAL STUDY OF THE MANTLING UNITS OF THE PELHAM GNEISS DOME, CENTRAL MASSACHUSETTS

Kinematic data and microstructural observations are used to assess the emplacement history of the Pelham gneiss dome. The Pelham gneiss dome is an elongate dome within a north-south trending belt of gneiss domes in the New England Appalachians. The core of the Pelham dome contains high-grade gneisses and intrusive rocks, and is mantled by Silurian and Devonian metasedimentary rocks. During the Paleozoic, the Pelham gneiss dome experienced several stages of metamorphism and deformation. A north-south oriented stretching lineation preserved in the dome core and in the mantling units is proposed to record late Pennsylvanian subhorizontal shearing. In order to evaluate the role of the late Pennsylvanian shearing, kinematic data and quartz microstructures were collected from the eastern margin of the dome, within the mantling cover sequence. A sequence of moderately strained Silurian Clough Quartzite and sillimanite orthoclase pegmatite were collected along a transect perpendicular to the metamorphic foliation. The foliation strikes northwest and dips shallowly to the northeast. The mineral stretching lineation, commonly defined by quartz, plunges shallowly to the north. Rocks in the Clough Quartzite contain quartz + muscovite + biotite + plagioclase + K-feldspar ± garnet and the pegmatite consists of sillimanite + orthoclase + quartz demonstrating high-temperature metamorphism. Deformation twinning in plagioclase and undulatory extinction in quartz are common. Asymmetric K-feldspar porphyroclasts, garnet porphyroblasts with asymmetric strain shadows, and rare mica fish indicate top-to-the-south shear sense. These observations are consistent with previous work in the northern portion of the Pelham dome. Within the quartzites, quartz engulfs mica grains suggesting possible grain boundary migration recrystallization. However, quartz-mica textures exhibit significant recrystallization that has overprinted much of the dynamic recrystallization. Samples analyzed using the SEM with EBSD display a moderate lattice preferred orientation. These kinematic data and microstructural observations––consistent with previous work––suggest that during ductile shearing the core of the Pelham dome moved northward relative to mantling cover units.